Gelatin in a layer-like form is employed as a binder in most photographic materials. Methods for hardening gelatin using a variety of compounds in order to increase the water resistance and mechanical strength of such gelatin layers were known in the past. It is important that the hardening reaction should be completed quickly when hardening the gelatin layers, and changes in the degree of hardness occur with the passage of time due to so-called after-hardening in cases where the hardening action on the gelatin is slow. It is well known that such after-hardening generates adverse effects (more precisely, increased fogging, a lowering of speed and a lowering of color density etc.) during the storage of photographic materials.
Moreover, the compounds which are used as hardening agents for gelatin must be highly soluble in water, and if their water solubility is low, then uneven hardening is liable to occur in the photographic emulsion layers, and special organic solvents must be used for the addition of the compounds to the photographic emulsion or photographic emulsion layers. Problems with uneven coating are liable to occur as a result of the presence of such solvents and care must also be taken to prevent explosions, etc.
Compounds for hardening gelatin in this way have been proposed for example in U.S. Pat. No. 3,642,486, Japanese Patent Publication No. 13563/74 and Japanese Patent Application (OPI) Nos. 73122/74 (corresponding to U.S. Pat. Nos. 3,841,872), 66960/78 (corresponding to U.S. Pat. Nos. 4,134,770) and 41221/78 (corresponding to U.S. Pat. No. 4,137,082) (the term (OPI) as used herein means a published unexamined Japanese patent application) where examples of the use of compounds which have a plurality of vinylsulfonyl groups within the same molecule as hardening agents for gelatin is disclosed.
However when these compounds are used as hardening agents for gelatin, there are problems with uneven hardening in the photographic emulsion layers since the compounds are all of low solubility in water, and the problem of uneven coating also arises because of the need to use organic solvents. Furthermore with some of these compounds the hardening reaction takes place slowly and the problem of "after-hardening" described earlier occurs.
An example of the use of compounds which have two vinylsulfonyl groups and sulfo groups in the same molecule as gelatin hardening agents is disclosed in Japanese Patent Application (OPI) No. 128240/86. These compounds have greatly improved water solubility, but the hardening reaction rate is inadequate in all cases, and so it is impossible to avoid the change in characteristics during storage of the photographic material which occur as a result of "after-hardening".